Nico is my preference. You may not get your desired pronunciation of "nee-ko" with the others (I'm inclined to say "neck-o" if spelled Neco or Neko). Niko reminds me of the Nike brand so I think Nico is the best. I love Nikolai nn Nico.

I think Nico is the most straightforward, followed by Niko. If I saw Neko or Neco I would wonder if it was pronounced Neh-ko instead. The only Neko I've ever heard of is NekoCase, and that's a woman. Also, Neko (pn neh-koh) is apparently Japanese for cat.